This invention relates to a method and apparatus for spatially disassembling signals, such as stereo audio signals, to produce additional signal channels.
In the field of audio, spatial disassembly is a technique by which the sound information in the two channels of a stereo signal are separated to produce additional channels while preserving the spatial distribution of information which was present in the original stereo signal. Many methods for performing spatial disassembly have been proposed in the past, and these methods can be categorized as being either linear or steered.
In a linear system, the output channels are formed by a linear weighted sum of phase shifted inputs. This process is known as dematrixing, and suffers from limited separation between the output channels. “Typically, each speaker signal has infinite separation from only one other speaker signal, but only 3 dB separation from the remaining speakers. This means that signals intended for one speaker can infiltrate the other speakers at only a 3 dB lower level.” (quoted from Modern Audio Technology, Martin, Clifford, Prentice-Hall, Englewood Cliffs, N.J., 1992.) Examples of linear dematrixing systems include:                (a) Passive Dolby surround sound.        (b) “Optimum Reproduction Matrices for Multispeaker Stereo,” Gerzon, Michael A., Journal of the Audio Engineering Society, Vol. 40, No. 7/8, July/August, 1992.        
Steered systems improve upon the limited channel separation found in linear systems through directional enhancement. The input channels are monitored for signals with strong directionality, and these are then steered to only the appropriate speaker. For example, if a strong signal is sensed coming from the right side, it is sent to only the right speaker, while the remaining speakers are attenuated or turned off. At a high-level, a steered system can be thought of as an automatic balance and fade control which adjusts the audio image from left to right and front to back. The steered systems operate on audio at a macroscopic level. That is, the entire audio signal is steered, and thus in order to spatially separate sounds, they must be temporally separated as well. Steered systems are therefore incapable of simultaneously producing sound at several locations. Examples of steered systems include:                (a) Active Dolby surround sound.        (b) Julstrom, Stephen, “A High-Performance Surround Sound Process for Home Video”, Journal of the Audio Engineering Society, Vol. 35, No. 7/8, July/August, 1987.        (c) U.S. Pat. No. 5,136,650, David H. Griesinger, Sound Reproduction.        
In order for a spatial disassembly system to accurately position sounds, a model of the localization properties of the human auditory system must be used. Several models have been proposed. Notable ones are:                Makita, Y., “On the Directional Localization of Sound in the Stereophonic Sound Field,” E.B.U. Rev., pt. A, no. 73, pp. 102-108, 1962.        M. A. Gerzon, “General Metatheory of Auditory Localisation,” presented at the 1992 Convention of the Audio Engineering Society, May 1992.        
No single mathematical model accurately describes localization over the entire hearing range. They all have shortcomings, and do not always predict the correct subjective localization of a sound. To improve the accuracy of models, separate models have been proposed for low frequency localization (below 250 Hz) and high frequency localization (above 1 kHz). In the range, 250-1000 Hz, a combination of models is applied.
Some spatial disassembly systems perform frequency dependent processing to more accurately model the localization properties of the human auditory system. That is, they split the frequency range into broad bands, typically 2 or 3, and apply different forms of processing in each band. These systems still rely on temporal separation in order to steer sounds to different spatial locations.